Oil from the Rena is expected to hit the shore this evening, with the slick set to extend further tomorrow.

Speaking in Tauranga this afternoon, Maritime New Zealand national on-scene commander Alex Van Wijngaarden said the oil had leaked into the water earlier today after the stern of the vessel sunk.

It was expected to wash ashore at Motiti Island by 6pm, he said. And it was likely the oil could hit Matakana Island and the shore of Matata.

By 10am today the stern had all but disappeared below the water, shortly before a crew from international salvage company Svitzer had planned to land on its deck.

Oil continued to leak into the surrounding waters.

Although the precise amount of leaked oil is unknown, Environment Minister Nick Smith said it was in 'single digits of tonnes'.

'The Rena is in her death throes,' Smith said.

Transport Minister Gerry Brownlee today visited Waihi Beach, where containers and debris from the Rena had washed up, and praised the clean up efforts of local volunteers and residents of the popular seaside holiday sport.

"They have tolerantly put up with all the inconvenience that comes from these types of events," he said.

And while clean up efforts continue, attention will soon shift to exactly what will happen to the now two separate parts of the ship.

"Our position at the moment is that the wreck will have to be removed," Browlee said.

It is now expected the Rena's stern, which held an estimated 400 containers, will have to be cut up on the sea floor.

The break up of the bow is seen as inevitable.

Oil spill response teams were gearing up to clean up the oil, including placing booms in sensitive areas, and the wildlife centre in Tauranga was being reactivated.

Rena grounded off the Tauranga coast more than three months ago, and split in two in rough weather at the weekend. By early afternoon the back section was perched on the edge of the Astrolabe Reef, about 75 per cent submerged with the rest sticking out of the water.

Grant Dyson, spokesman for container recovery company Braemar Howells, said two tugs had been sent to the Rena to try to contain drifting containers. They would be towed to a specialised recovery barge that was being deployed.

New exclusion zones have been put in place around the sinking ship.

TOTAL BREAK UP INEVITABLE

The bow section remains on the reef but Smith said it was now inevitable that would break up too. The Government expected the wreck would be removed.

It was the responsibility of the shipping company to remove the wreck and it was technically possible to do so.

There were three pollution risks, Smith said: oil in the keel, the release of the cargo and containers and the threat to navigation.